The present invention relates to the field of implantable cardiac devices and, in particular, to a small permanent magnet provided with a displaceable, conformal magneto-static shield to inhibit unintentional exposure of the magnetic field that is suitable for carrying on the person.
Cardiac devices are known assemblies implanted in patients to monitor the heart and provide therapeutic stimuli to treat a variety of arrhythmias. Many of these devices also have features that may be selectively activated by exposure to a magneto-static field. Typically, these devices include well-known reed switches that can be closed by exposure to a magnetic field of a given threshold value. A typical use of a magnetically activated reed switch in an implanted cardiac device is to enable a telemetry circuit within the device so that data indicative of the function of the patient""s heart, as it is sensed by the implanted cardiac stimulation device, as well as data indicative of the function of the implanted device can be telemetered to an external programmer. This data can be reviewed by a treating medical professional. The advantage of using a magnetically activated switch in this circumstance is that it permits the selective activation of a particular function of the device that is implanted within the patient in a simple, non-invasive manner.
Magneto-static fields are chosen to activate these selectable features for several reasons. A patient is not likely to encounter strong magneto-static fields ( greater than 0.5 Gauss) inadvertently. Magneto-static fields pass relatively readily through the body and thus to the implanted device. Magneto-static fields of reasonable strength have no known injurious effects on the human body. A small, high strength permanent magnet can be readily carried on the person and used by the patient to activate the selectable features of the cardiac device when desired.
However, several problems occur with carrying a permanent magnet on the person. If the magnet is inadvertently brought too close to the device, the selectable features of the device can be unintentionally activated. Also, strong magneto-static fields can irreparably scramble data stored on magnetic recording media. In fact, exposure to high gauss fields is a known manner of wiping magnetic recording media, such as computer diskettes, audio tapes and the like. Credit cards are also typically provided with magnetic strips with account holder information encoded therein and exposure to a permanent magnet can erase this information from the card.
An additional liability to permanent magnets carried on the person is that they are attracted to and can adhere to ferrous material. For example, a magnet carried in the person""s pocket can be attracted and stick to a steel structure. It will be appreciated that a magnet, unexpectedly adhering to a steel railing on a stairway, for example, could induce a person to stumble and fall, possibly leading to injury. A permanent magnet would also be attracted to ferrous items such as keys, pocketknives, pens, and fingernail files that are often carried in a purse or pocket. A magnet could further attract and knock over steel objects such as cans, medical instruments, etc. as a person carrying a magnet walks by.
In addition, exposing certain materials, the most common of which are ferrous materials, to a magnetic field causes the materials so exposed to become magnetized themselves. Thus a steel key and key ring, for example, placed in proximity to a permanent magnet, would become partially magnetized themselves and would have similar characteristics to those of the original magnet.
A further difficulty that occurs with these magnets in connection with implantable cardiac devices is that the unshielded magnets are strong enough to result in inadvertent activation of the reed switches in an implanted device while the medical professional carrying the magnet is in the presence of the patient. This can result in undesired operation of the device resulting in undesired drain of limited battery resources. Moreover, the magnets are also strong enough that the magnets can affect the operation of external programmers that are used to evaluate the operation of the cardiac stimulation device implanted within the patient.
Unfortunately, while these magnets are necessary to permit remote activation of functions within the implanted cardiac stimulation device, there is no way to deactivate the magnets. Hence, the problems associated with carrying around magnets of sufficient strength to activate functions within an implanted cardiac stimulation device have not been readily addressed in the prior art.
From the foregoing it will be appreciated that there is an ongoing need for a small, permanent magnet that can be readily carried on a person to enable a person implanted with a cardiac stimulation device to employ the magnet to selectively activate certain features and functions of the implanted device. Moreover, there is still an ongoing need to develop a magnet device suitable for activation of magnetic switches in implanted cardiac devices that can also be shielded when the magnet device is not being used to avoid the difficulties associated with medical professionals carrying around powerful magnets.
The aforementioned needs are satisfied by the magnet device of the present invention which in one aspect is comprised of a magnet and a configurable container. The magnet can be exposed wherein it produces a magnetic field of a first strength sufficient to activate a magnetic switch within an implanted cardiac stimulation device to thereby induce the implanted cardiac stimulation device to perform a selected function. The magnet can also be shielded within the container such that the magnet produces a field of second strength that is sufficiently less than the first strength such that the magnet does not activate the magnetic switch within the implanted cardiac stimulation device.
Preferably, the container defines a high magnetic permeability path through which a substantial portion of the flux flows to thereby reduce the strength of the magnetic field outside the container. Preferably, the container is made of a material that has a high level of magnetic permeability. Magnetic permeability in the context of magnetic fields is analogous to electrical conductivity in the context of electrical current. Given alternative paths with high and low conductivity, electrical current will predominantly flow through the path with high conductivity (low resistance). In a similar manner, magnetic fields will predominantly pass through regions of high permeability in preference to regions of low permeability. Air and most common materials have relatively low permeabilities on the order of 1. However, materials such as iron and MuMetal(copyright) have permeabilities on the order of tens of thousands. Thus, in one embodiment, if the container has sufficient quantities of high permeability material that is placed about the permanent magnet, the magnetic field will predominately pass within the highly permeable path and thus reduce the magnetic field strength induced by the permanent magnet outside of the container. Advantageously, the high permeability material does not damage magnetic field strength, it is simply providing a more permeable path for the magnetic flux in the container material.
In one embodiment, the magnet device produces a magnetic field of at least 10 Gauss measured 7.6 cm from the magnet. When the magnet is shielded within the container, in this embodiment, the magnet produces a magnetic field of less than 2 Gauss measured 7.6 cm from the magnet.
The container can have a variety of different configurations. The magnet can be positioned within a container such that it can be removed from the container. The magnet can also be fixedly mounted within the container and a lid of the container can be removed or the magnet can be otherwise exposed to produce the larger magnetic field.
In another aspect, the magnetic device can include an electromagnet assembly for selectively activating features of an implanted cardiac stimulation device. In this aspect, the magnet device is electrically actuated to produce a stronger magnetic field having a magnetic field strength sufficient to activate a magnetic switch in an implanted device. When the device is not actuated, the magnetic field strength is low enough not to result in activation of the magnetic switches and also reduces the inconvenience of having a strong magnet in the presence of other metal objects.
The present invention therefore provides a mechanism that reduces the negative effects of magnetic fields emanating from magnets that are used to activate selected functions of implanted medical devices, such as implanted cardiac stimulation devices. These and other objects and advantages will be more apparent from the following discussion taken in conjunction with the accompanying drawings.